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https://hdl.handle.net/2142/72276
Description
Title
Electrochemical Studies at High Pressure
Author(s)
Cruanes, Maria Teresa
Issue Date
1993
Doctoral Committee Chair(s)
Faulkner, Larry R.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Chemistry, Physical
Abstract
This research has dealt with the development and application of a methodology that permits electrochemical measurements at high pressure. The utilization of this methodology has proven useful in the characterization of various systems and processes as briefly outlined below.
Our initial efforts focused on the design and construction of an electrochemical cell functional at hydrostatic pressures as high as 10 kbar. This cell was equipped with an Ag/AgCl/KCl (0.1M) reference electrode which provides reliable control of the potential at all pressures. The potential of this reference electrode can be considered to be, for most purposes, constant with pressure. According to our estimation, at the highest pressure, 10 kbar, the uncertainty in the constancy of the potential of the reference electrode, with respect to the ambient pressure value, is only about $\pm$15 mV. Measurements of formal potentials (E$\sp{\circ\prime}$) of several transition-metal complexes vs the Ag/AgCl electrode rendered volumes of reactions whose magnitudes support our prediction of the negligible pressure dependence of the reference electrode. The high-pressure behavior and volumes of reaction that we observed for these complexes can be rationalized in terms of solvation effects.
The main systems that we have investigated at high pressure are surface-modified electrodes. First, we studied the effect of compression on the dynamics of charge transport in quaternized poly(4-vinylpyridine) (QPVP) films placed on gold electrodes, loaded with potassium ferricyanide, and equilibrated in potassium nitrate. Pressure accomplished the continuous change in the structure of the polymer network. This change, which we attributed to dehydration of the film, causes a pronounced restriction in the propagation of charge and in the motion of mass. Our results are in agreement with a model in which physical diffusion of the redox centers, the ferri/ferrocyanide ions, is the principal mechanism responsible for electron transport in the QPVP film.
This high-pressure methodology has also allowed the spatial characterization of electron transfer events taking place between a gold electrode and ferrocene molecules covalently attached to the end of 1-undodecanethiol chains self-assembled on the electrode surface. The volumes of reaction and activation for the oxidation process are both positive, indicating that a volume expansion is associated with the formation of ferricinium. To interpret these results, we propose a model in which the creation of a vacancy in the self-assembled monolayer, for the accommodation of the ferricinium ion or a charge-compensating anion, is coupled with the electron transfer step.
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